If $e$ is the eccentricity of the ellipse $\large\frac{x^2}{a^2}+\frac{y^2}{b^2}\normalsize =1\;(a < b)$,then

Question

$\begin{array}{1 1}b^2=a^2(1-e^2)\\a^2=b^2(1-e^2)\\a^2=b^2(e^2-1)\\b^2=a^2(e^2-1)\end{array} $